PageConverter::update_header(): Remove an unnecessary write.
The field that was originally called FIL_PAGE_FILE_FLUSH_LSN only
made sense for the first page of the system tablespace
(initially, for the first page of each file of the system tablespace).
It never had any meaning for .ibd files, and it lost its original
meaning in MariaDB Server 10.8.1 when
commit b07920b6 (MDEV-27199)
removed the ability to start without ib_logfile0.

If the most significant 32 bits of the LSN are nonzero, this
unnecessary write would write the wrong encryption key identifier
to the page. The first page of any file is never encrypted,
so normally those bytes should be 0 for any .ibd file.